This invention relates generally to antenna elements and in particular to a broadband waveguide antenna element suitable for use in antiradiation missile seeker applications.
Manned aircraft have been, and will continue to be, one of the principal means of weapons delivery in modern warfare. Manned aircraft combine a capability for accurate delivery of projectiles with the capability of reconnaissance and surveillance, utilizing personnel within the aircraft for location and identification of ground targets. Improved radar processing techniques, such as synthetic aperture mapping, then may be used to supplement the senses of the personnel to provide capability of attacking ground targets under adverse weather conditions and at night. Therefore, if allowed to roam freely in the airspace over the battlefield, manned aircraft can be the decisive factor in any ground engagement.
To counter the threat posed by manned aircraft, highly effective ground-based anti-aircraft defense systems are being developed and deployed to reduce the probability of penetration of battle areas by manned aircraft to make the sustained use of such aircraft impractical. The common feature of such systems is the use of some form of radiation, such as radar, for the functions of search, acquisition, tracking or fire control of airborne vehicles, including manned aircraft. One most effective way to counteract the systems being discussed is, of course, to provide guided missiles which, when launched from an aircraft, sense the radiation and home in on the source of such radiation to deliver appropriate ordnance to such source. The radiation from the aforementioned defense systems may lie at any frequency within a wide frequency band and because such radiation may have one of several polarization senses, a missile seeker designed to home in on the source of such radiation (sometimes hereinafter referred to as an antiradiation missile (ARM) seeker) must be capable of operating over a similarly wide band of frequencies and must be responsive to any one of several polarization senses.
An antenna including a matrix of stripline tapered notch elements, as described in an article entitled, "A Broadband Stripline Array Element," by L. R. Lewis, M. Fassett and J. Hunt, IEEE Antenna Propagation Society Symposium at Atlanta, Ga., June 1974, has been developed for ARM seeker applications. In such elements, a notch is etched away on both ground planes of a stripline and the stripline center conductor is arranged to excite a voltage across the notch. The matrix of elements is mounted orthogonally with respect to a feed network, requiring a right angle transition to be made between each element and the feed network. Such right angle transitions are extremely difficult to match over a wide frequency band with the result that impedance mismatches between the elements and the feed network may seriously degrade the seeker antenna performance. In addition, the physical size of such elements and the manner in which the matrix of such elements is mounted orthogonally to the feed network makes vibrational damage in missile seeker applications quite likely.
The gain of the stripline tapered notch element being discussed is a function of the length of the element. Thus, an element having a length of approximately one-half wavelength at the highest operating frequency has a gain of from 1 to 3 db over an octave band, while an element of the same width but having a longer notch has greater than 4 db of gain over the same frequency band. In instances where space available for an antenna does not allow long elements, as in missile seeker applications, the gain of such elements is limited.
Thus, there exists a need for a broadband antenna element, suitable for missile seeker applications, which may be integrated to a stripline feed network without causing severe mismatch problems and which is not susceptible to vibrational damage.